Cannot use set_memory_rw in Linux kernel on ARM64

2 min read 06-10-2024

The "Cannot use set_memory_rw in Linux Kernel on ARM64" Mystery: Unlocking the Secrets

Have you ever encountered the frustrating error "Cannot use set_memory_rw in Linux kernel on ARM64" while working on your ARM64 device? This error, often seen during kernel development or debugging, can leave you scratching your head. Let's dive into the core of this problem and unlock its secrets.

The Problem: Why Can't We Just Change Memory Permissions?

The set_memory_rw function, present in the Linux kernel, aims to make a specific memory region writable. This is often necessary during debugging to examine and modify memory contents directly. However, on ARM64 systems, using set_memory_rw is generally not recommended and can lead to the dreaded "Cannot use set_memory_rw..." error.

The Original Scenario and Code: A Glimpse into the Issue

Imagine you're debugging a kernel module on your Raspberry Pi 4 (an ARM64 device). You attempt to use set_memory_rw to modify a memory region containing sensitive data. The kernel, however, throws the "Cannot use set_memory_rw..." error, preventing your debugging attempt.

#include <linux/kernel.h>
#include <linux/module.h>

static int __init my_module_init(void)
{
    // Example: Attempt to modify a memory region
    void *mem_addr = (void *)0x1000;
    set_memory_rw(mem_addr, PAGE_SIZE); 
    // ... rest of your code ...

    return 0;
}

static void __exit my_module_exit(void)
{
    // ... cleanup code ...
}

module_init(my_module_init);
module_exit(my_module_exit);

MODULE_LICENSE("GPL v2");

The Root Cause: ARM64 Memory Protection & Security

ARM64 architectures, known for their performance and power efficiency, prioritize strong memory protection. Unlike some older architectures where direct memory manipulation was prevalent, ARM64 emphasizes security and stability. This leads to the "Cannot use set_memory_rw..." error because:

Hardware Limitations: Direct memory manipulation can easily introduce vulnerabilities and security risks on ARM64. The architecture restricts such actions to protect the system integrity.
Kernel Restrictions: The Linux kernel for ARM64 is designed with these security considerations in mind. It enforces restrictions on functions like set_memory_rw, preventing their use for most scenarios.

The Solution: Navigating the Restrictions

While directly using set_memory_rw is discouraged, there are effective alternatives to achieve your debugging needs:

Use Debugging Tools: Leverage powerful kernel debugging tools like kgdb or kprobe to examine and modify memory contents in a controlled and secure manner. These tools provide a safer and more controlled environment for debugging than direct memory manipulation.
Utilize kmalloc and kfree: Instead of modifying existing memory regions, allocate new memory with kmalloc and free it with kfree. This approach ensures you work within the kernel's memory management framework, enhancing security and stability.
Implement Secure Debugging Strategies: Consider utilizing advanced debugging techniques like:
- Memory Breakpoints: Set breakpoints on specific memory locations using kprobe to trigger debugging actions when memory is accessed.
- Kernel Tracing: Use kernel tracing tools like trace-cmd or perf to capture detailed information about memory access patterns and pinpoint issues.

Conclusion: Embracing the Secure Approach

The "Cannot use set_memory_rw in Linux kernel on ARM64" error isn't a dead end, but a reminder that the ARM64 architecture prioritizes security. Embrace the alternative debugging methods mentioned above to continue your development and exploration journey on ARM64 platforms.

Remember: Always prioritize security and stability over direct memory manipulation, especially when working on embedded systems.

For more detailed information on debugging ARM64 Linux kernels, refer to the official documentation and online resources.